FIGS. 1-6 is a prior art
FIG. 1 shows a prior art of a pressure gauge
A section view of the traditional pressure gauge is shown. A piezoresistor is made of a top stack TS and a bottom stack BS. A spacer 15 is inserted in between the two stakes in the periphery to make a center space 16 in between the two stacks.
The top stack TS includes sequentially from top to bottom: a top substrate 10, a top metal electrode 11, and a top piezoresistive layer 12. The bottom stack BS includes sequentially from top to bottom: a bottom piezoresistive layer 129, a bottom metal electrode 11, and a bottom substrate 109. A spacer 15 is inserted to form a center space 16 in between the top piezoresistive layer 12 and the bottom piezoresistive layer 129. The top metal electrode 11 electrically couples to a first electrode of the electronic system 13, and the bottom electrode 129 electrically couples to a second electrode of the electronic system 13.
FIG. 2 is an initial status of the prior art
When a pressure is applied to the pressure gauge 100 initially, the top piezoresistive layer 12 bends down to touch the bottom piezoresistive layer 129. Just before touching, an initial thickness L1 is a total thickness of the top piezoresistive layer 12 plus the bottom piezoresistive 129. An output resistance can be calculated according to ohm's law as: R1=ρL1/A1. At initial status, the contact area A1 at point P1 approaches zero. Therefore, the output resistance R1 is calculated to be infinite as follows:R1→∞ when A1→0.
FIG. 3 is a stable status under pressure of the prior art
The pressure gauge 100 is pressed further so that the piezoresistive layers 12, 129 are compressed and the total thickness L2 of the two piezoresistive layers 12, 129 becomes lesser than the initial thickness L1. In the meanwhile, the contact area A2 at area P2 is larger than the initial contact area A1. At this moment, the output resistance is calculated as follows:R2=ρL2/A2∘
FIG. 4 is pressure tests of the prior art
Three different points P1, P2, and P3 are chosen to be tested in a prior art pressure gauge 100. Point P1 is the center of the pressure gauge 100, point P2 is a little far away from the center point P1, and point P3 is even farther away from the center point P1. Various pressures are applied to each of the three points for checking the corresponding conductance, the conductance--pressure curves are then made as shown in FIG. 5.
FIG. 5 is the conductance--pressure curves for points P1, P2, and P3
The top line is for point P1, the middle line is for point P2, and the bottom line is for point P3. The curve for P1 has a largest slop, the curve for P2 has a less slop, and the curve for P3 has a least slop. The curve slop is lesser as the test point farther away from the center. In other words, the farther a test point is away from the center, the less precision it becomes. Further in other words, different conductance can be obtained when a same pressure is applied at a different point of a traditional pressure gauge 100. Curve P1 has the best identification ability, curve P2 has moderate identification ability, and curve P3 has the worst identification ability. The position dependent curve--pressure feature makes the prior art pressure gauge 100 unreliable, unless a fixed test position is used. Take an example to see different conductance is obtained for a same pressure: a conductance of 6.5*10exp (−4)/ohm is obtained for curve P1 at 20 psi; a conductance of 3.5*10exp (−4)/ohm is obtained for curve P2 at 20 psi; and a conductance of 2.8*10exp (−4)/ohm is obtained for curve P3 at 20 psi. Serious problems shall be caused if the prior art pressure gauge 100 is designed in a weighing machine with parallel connection. It becomes a big challenge as how to design a correction circuit to modify the deviation in order to obtain a linear output in order for realizing a weighing machine with the traditional pressure gauge 100.
FIG. 6 is a pressure test with prior art pressure gauge.
FIG. 6 shows when a product Wt with a rugged bottom is put on a parallel connected prior art pressure gauges 101, 102, 103 which are configured on a substrate 209. As shown in the figure, the rugged bottom of the product Wt touches point P1 of the pressure gauge 101, touches point P2 of the pressure gauge 102, and touches point P3 of the pressure gauge 103. It is difficult to obtain an accurate weight from the prior art pressure gauge 100 because of the non-consistent conductance-pressure curve for different points P1, P2 and P3.
Now, please refer to FIG. 3. The basic principle for the prior art follows the Law of Resistance R=ρL/A, the changes of the total thickness L, and the changes of the touching area A between the two piezoresistive layers 12, 129 are two determinants for the output resistance R. Therefore, the prior art pressure gauge needs to consider the two factors when a pressure is applied, and especially when a pressure is applied on partial area instead of full surface of the pressure gauge 100. Further more, an anti-pressure of the spacer 15 in the peripheral is another headache problem needs to be overcome for the prior art pressure gauge 100. A single conductance--pressure curve for a pressure gauge independent of position with a stable and reproducible output is desired for a long time.